1. Field of the Invention
The present invention relates to a picture coding apparatus composing a terminal apparatus in which data including picture data is transmitted and received.
2. Description of Related Art
An example of a conventional picture coding apparatus using a H.263 coding method (ITU-T Recommendation) is shown in FIG. 1.
In a motion compensation predicting unit 27, referential picture data of a memory unit 19 and input picture data are received, a block matching operation is performed for each macro-block (16 pixelsxc3x9716 lines), and motion information is obtained. Thereafter, in the motion compensation predicting unit 27, referential picture data of a position corresponding to the motion information is received from the memory unit 19, and a predicted picture is produced. The predicted picture produced in the motion compensation predicting unit 27 is output to a subtracting unit 13 and a locally-decoded picture producing unit 18.
In the subtracting unit 13, a differential picture obtained from the input picture data and the predicted picture is produced for each macro-block.
Thereafter, in a coding mode judging unit 14, a coding mode (intra/inter) is selected according to a prescribed method for each macro-block. In cases where an intra-coding mode is selected, picture data of one macro-block is output to a discrete cosine transforming (DCT) unit 15. In contrast, in cases where an inter-coding mode is selected, the differential picture of one macro-block is output to the DCT unit 15. The selected coding mode is output to the locally-decoded picture producing unit 18.
In the DCT unit 15, the differential picture input of the macro-block is divided into a plurality of differential pictures respectively corresponding to one block (8 pixelsxc3x978 lines), and a DCT processing is performed for the differential picture for each block of the macro-block. The differential pictures, for which the DCT processing is performed, are output to a quantizing unit 16.
In the quantizing unit 16, a quantization processing is performed for each differential picture according to a prescribed method, quantized DCT coefficients (hereinafter, expressed by a group of quantization indexes) are output to a variable length coding (VLC) unit 20 and a locally-decoding prediction error signal producing unit 17.
In the locally-decoding prediction error signal producing unit 17, an inverse quantization processing and an inverse DCT processing are performed for the group of input quantization indexes, and a locally-decoding prediction error signal is produced. The produced locally-decoding prediction error signal is output to the locally-decoded picture producing unit 18.
The locally-decoded picture producing unit 18 is operated according to the coding mode for each macro-block. In cases where the coding mode is the intra-coding mode, the input locally-decoding prediction error signal is written in the memory 19 as a locally-decoded picture. In contrast, in cases where the coding mode is the inter-coding mode, the locally-decoding prediction error signal and the predicted picture are added to each other to produce a locally-decoded picture, and the locally-decoded picture is written in the memory 19.
In the VLC unit 20, header information and the group of quantization indexes are coded according to the H.263 syntax, and a multiplexed bit stream is output.
In the above header information, a synchronization signal is included for each prescribed interval corresponding to a prescribed number of blocks, and a differential motion vector (which denotes the difference between a motion vector of a corresponding macro-block and a motion vector of a macro-block adjacently placed on the left side of the corresponding macro-block) is included for each macro-block.
FIG. 2 shows an example of the bit stream output from the VLC unit 20. In this example, header information including one motion vector is added to the bit stream for each group of quantization indexes corresponding to one macro-block, and one synchronization signal is inserted into the header information every eight macro-blocks. That is, one synchronization signal is inserted into the bit stream every eight pieces of header information.
However, as is described above, in the conventional picture coding apparatus, a picture coding processing is performed regardless of quality conditions of a data transmission line through which media data including compressed picture data is transmitted. Therefore, in cases where the quality conditions of the data transmission line become worse because of some causes, it becomes impossible to play back any type of media data on the receiving side. As a result, there is a problem that the media data playback fails when the media data decoded result is displayed.
Therefore, to make the playback of any type of media data possible on the receiving side regardless of the quality conditions of the data transmission line, an object of the present invention is to provide a picture coding apparatus in which compressed picture data is output while performing the control of an amount of bits of the compressed picture data, the control of insertion intervals of synchronization signals and the control relating to insertion intervals of intra-coded blocks.
To achieve the above object in a present invention, a picture coding apparatus composing a terminal apparatus, in which data including picture data is transmitted and received, is characterized in that the picture coding apparatus comprises differential motion information threshold value determining means for setting a threshold value, which gives a value range to differential motion information, according to a quality condition of a given line, and outputting the threshold value; and
picture data coding means for performing a motion compensation processing for input picture data and reproduced picture data obtained by performing a coding processing and a decoding processing for preceding picture data precedent to the input picture data to obtain motion information, calculating a difference between the motion information and preceding motion information obtained in a preceding motion compensation processing precedent to the motion compensation processing as the differential motion information, producing a renewed differential motion information by setting a limit to the differential motion information according to the threshold value set by the differential motion information threshold value determining means in cases where the differential motion information is beyond the value range given by the threshold value, coding the input picture data and outputting compressed picture data.
Also, to achieve the above object in a next present invention, a picture coding apparatus composing a terminal apparatus, in which data including picture data is transmitted and received, is characterized in that the picture coding apparatus comprises control signal setting means for outputting a control signal indicating a first mode, at which the picture data quantized in a frequency area is coded, or a second mode, at which the picture data quantized in a frequency area is not coded, according to a quality condition of a given line; and
picture data coding means for coding the input picture data and header information in cases where the control signal output from the control signal setting means indicates the first mode, coding only the header information in cases where the control signal indicates the second mode, and outputting compressed picture data.
Also, to achieve the above object in a next present invention, a picture coding apparatus composing a terminal apparatus, in which data including picture data is transmitted and received, is characterized in that the picture coding apparatus comprises code-word limiting means for setting a threshold value, which gives a value range of a code length to a variable length code-word obtained in an entropy coding of picture data for which an information source coding is performed, according to a quality condition of a given line, and outputting the threshold value; and
picture data coding means for setting a limit to the input picture data, for which an information source coding is performed, to select a specific variable length code-word having a code length, which is placed within the value range given by the threshold value output from the code-word limiting means, in cases where a code length of an original variable length code-word, which is obtained by performing an information source coding and an entropy coding for the input picture data, is beyond the value range given by the threshold value, multiplexing the specific variable length code-word, for which the entropy coding is performed, with compressed picture data, and outputting multiplexed and compressed picture data.
Also, to achieve the above object in a next present invention, a picture coding apparatus composing a terminal apparatus, in which data including picture data is transmitted and received, is characterized in that the picture coding apparatus comprises synchronization signal insertion interval determining means for setting information which indicates a time period at which a synchronization signal is repeatedly inserted, according to a quality condition of a given line, and outputting the information; and
picture data coding means for performing a frequency transforming processing and a quantization processing for the input picture data, inserting a synchronization signal into the compressed picture data for each time period indicated by the information output from the synchronization signal insertion interval determining means, when the input picture data is coded and multiplexed in a frequency area for which the quantization processing is performed, and outputting compressed picture data.
Also, to achieve the above object in a next present invention, a picture coding apparatus composing a terminal apparatus, in which data including picture data is transmitted and received, is characterized in that the picture coding apparatus comprises the-number-of-intra-macro-blocks determining means for setting the-number-of-intra-macro-blocks information, which indicates the number of intra-macro-blocks to be intra-coded, according to a quality condition of a given line, and outputting the the-number-of-intra-macro-blocks information; and
picture data coding means for setting a coding mode for one or more specific macro-blocks, which is selected from a plurality of macro-blocks composing the input picture data and of which the number is indicated by the the-number-of-intra-macro-blocks information output from the the-number-of-intra-macro-blocks determining means, to an intra-coding mode, coding the plurality of macro-blocks composing the input picture data while intra-coding the specific macro-blocks, and outputting compressed picture data.